1. Field of the Invention
The present invention relates to a rotor of an electric motor with magnets which are attached to an outer circumferential surface of a rotor core, an electric motor, and a method of producing a rotor of an electric motor.
2. Description of the Related Art
In general, an electric motor of a type which has permanent magnets attached to the outer circumferential surface of a rotor core is called an SPM (surface permanent magnet) motor. In relation to this, JP-A-2013-165548 describes a rotor of an SPM motor which has segment type permanent magnets fixed by a thermosetting binder to the outer circumferential surface of a rotor core. In this regard, a conventional SPM motor employs either a magnet with a confronting face toward the rotor core which is formed of a curved surface to follow the outer circumferential surface of the rotor core, or a magnet with a confronting face which is formed of a flat surface. The SPM motor of JP-A-2013-165548 employs the former magnet, but production of such magnet requires an additional process for forming the curved shape confronting face. Therefore, from the viewpoint of the manufacturing costs, magnet with flat confronting face may be advantageous.
However, accurately placing a magnet with flat confronting face at the rotor core requires formation in advance of guide projections for positioning the magnets at the outer circumferential surface of the rotor core. FIG. 10 is a perspective view which shows a conventional rotor R which has a plurality of guide projections P for positioning a plurality of magnets M. For convenience, FIG. 10 shows only a single magnet M, and the remaining magnets M are omitted. Further, FIG. 11 is an enlarged, cross-sectional view along a plane vertical to a rotation axis RA of the rotor R of FIG. 10, which shows one magnet M and its vicinity. As shown in FIG. 10, the outer circumferential surface of the rotor core C in a conventional rotor R is formed with a plurality of guide grooves G which are arranged at equal intervals in the circumferential direction. Each of the guide grooves G extends along the entire length in the height direction of the rotor core C.
Further, a guide projection P is provided between each two adjoining guide grooves G, G. To form such guide grooves G, groove cutting has to be performed on the entire length of the rotor core C, and therefore the machining man-hour increase and manufacturing costs may also increase due to the increased machining man-hour. Such increased manufacturing costs are particularly prominent in the case where the electric motor has a large number of poles. Further, if the magnets M are attached to the rotor core C after being magnetize, the flat confronting faces CS of the magnets M will closely contact the flat bottom surfaces BS of the guide grooves G due to the attraction force, and therefore, as shown in FIG. 11, the binder B will end up being removed from between these surfaces CS and BS. As a result, the bonding strength acting on the magnets will fall, and therefore the centrifugal force which accompanies rotating motion of the rotor R is liable to cause the magnets M to be scattered from the rotor core C.
A rotor of an electric motor which can reduce the machining man-hour for forming projections for positioning magnets has therefore been sought.